Internal Field.check() methods don't need to construct and return lists

[Chris Jerdonek]

I noticed that many of the internal "check()" methods used by Field subclasses return lists (including the empty list) when they can just yield items (or yield nothing). This is because the concrete check() implementations unpack the return value when constructing a new list. See here for one example of such a check() method: ​https://github.com/django/django/blob/f331eba6d576752dd79c4b37c41d981daa537fe6/django/db/models/fields/__init__.py#L196-L205

The proposed change, then, would be to change methods like the following to yield their items instead of creating and returning a list: ​https://github.com/django/django/blob/f331eba6d576752dd79c4b37c41d981daa537fe6/django/db/models/fields/__init__.py#L243-L254